The method for creating various patterns such as images, fonts, and two-dimensional barcodes on a surface of or inside a medium using laser is referred to as laser marking. Laser marking is divided into that for creation on a medium surface and that for creation inside a transparent medium such as glass. Lasers used for laser marking include nano-second lasers that output pulses with a pulse width of 10−9 second or more, pico-second lasers and femto-second lasers that output ultrashort pulses with a smaller pulse width, and so on. The following technology has been reported as an example of creation inside a transparent medium such as glass.
Patent Document 1 shown below discloses a technology for forming crack-shaped micro-denatured regions by concentrating a nano-second pulsed laser inside a medium having transparency to laser wavelengths. In the same document, the size of the micro-denatured regions is controlled by changing the number of pulses irradiated to record a grayscale image.
Patent Document 2 shown below discloses a method for preparing a plurality of layers in which a color caused by a color center or fine silver particle develops by irradiating glass with a femto-second laser and for changing a grayscale or hue by changing the number of stacked layers. Also, the document describes that there is a possibility that a full-color image can be expressed by using the method described in the same document (refer to paragraph 0032).